1. Field of the Invention
The present invention relates to hydraulic actuators for engine valves. More specifically, the present invention relates to hydraulic actuators for engine valves where both a three-way and a four-way actuator is used to open and close at least two exhaust valves per engine cylinder.
2. Description of the Prior Art
It is known to use hydraulic power in some type of electronically controlled actuator to open an engine valve without using a camshaft as may be seen by reference to U.S. Pat. Nos. 4,200,067; 4,974,495; 5,197,419; 5,221,072; and 5,275,136; the disclosures of which are hereby expressly incorporated by reference. The hydraulic activation is supplied by high pressure oil from an axillary pressure source such as a pump, and in some cases, a low pressure source is also used. Electronically controlled solenoid or spool valves open and close hydraulic ports to control the flow of hydraulic oil in and out of one or two chambers on either side of a working piston which is connected to the engine valve.
During operation, the engine exhaust valve in each cylinder must open against a residual cylinder pressure remaining after the power stroke of a four-cycle engine. Approximately 150 pounds per square inch of pressure remains in the cylinder when the exhaust valve is required to open to allow the combustion products to be pushed out of the cylinder by the piston. After the exhaust valve opens, the residual pressure quickly subsides as the gas products flow out the open exhaust valve, and the piston commences the exhaust stroke to further clear the cylinder of combustion products.
Initially, the hydraulic actuator must have the capability to open the exhaust valve against this relatively high cylinder pressure acting against the cross-sectional area of the exhaust valve tending to hold it closed. The intake valve does not have to be opened against cylinder pressure so the force capability of the hydraulic actuator operating the intake valve can be at a lower level. Traditionally, a higher pressure for the first to open exhaust valve has been used to ensure a high probability of opening the exhaust valve at the desired time, and high energy consumption results because of the increased hydraulic losses at higher pressure.